I Know Why the Caged Bird Sings is challenged pretty often in public schools as having inappropriate content for high school readers. There are some sexual references which some consider too graphic, including rape, premarital sex and same sex relationships. Others think there is too much violence (I didn't think so), and some feel the way in which the book deals with racial issues "encourages bitterness towards white people" (Alabama).
As an educator, I have found that any time a book contains mature themes, or content that some find controversial, it is challenged in some way. School Boards are the most common place a book is banned, and these groups tend to be more reactionary towards literature anyway, and often are made of up religious and social conservatives who are more concerned with what people read than most Americans.
Personally, I make it a point to read banned books.